Modern turboprop and turbofan powered aircraft carry a small gas turbine engine known as an APU in addition to their main propulsion gas turbine engines. Some aircraft carry two APUs. Typically, the APUs are located in a compartment located within the aft section of the aircraft, such as in the tail, forward of the tailcone. APUs have historically been used in aircraft to provide power to aircraft systems when the main engines are not running and to enable starting the main engines without need for external power. Aircraft also need an additional source of power while in flight. Further detail concerning the components and operation of an exemplary tail mounted APU may be found in co-owned, re-issued patent 39,972, which is hereby incorporated by reference in its entirety.
During operation of the APU, heat is generated in the APU compartment from numerous sources including radiation from the APU, convection from an APU oil cooler, and from a generator and other auxiliary equipment. To prevent the temperature in the APU compartment from reaching unacceptable levels, a ventilating or cooling airflow must be provided through the compartment. To provide this cooling airflow, an exhaust eductor has been used. The eductor uses the kinetic energy of the APU exhaust gas to create a modest vacuum in the tailcone to pull ambient cooling airflow through the APU compartment and over the APU oil cooler. Further background detail concerning an exemplary exhaust eductor cooling system may be found in co-owned U.S. Pat. No. 5,265,408, which is hereby incorporated by reference in its entirety.
FIG. 1 is an exemplary prior art diagram of a conventional ITM 100 for the Comac C919 and similar aircraft. The Comac C919 is a 168-190 seat narrow-body airliner to be built by the Commercial Aircraft Corporation of China. The ITM 100 is disposed within an aircraft tailcone 2 having an auxiliary power unit 12 mounted in a compartment forward thereof. The tailcone 2 is generally conical in shape comprising a tapered hollow shell with an aft end (i.e., an apex) that is truncated and opens to the ambient atmosphere (hereinafter “open aft end”)
The auxiliary power unit 12 and oil cooler plenum 17 are disposed forward of the tailcone 2. The APU oil cooler 10 and its respective oil cooler plenum 17 are fluidly coupled to an inlet opening of the ITM 100. A surge bleed plenum 99 is also fluidly coupled to the inlet opening of the ITM 100. During normal APU operation exhaust entrains cooling air through the APU compartment, over AU oil cooler 10 and into the cooling plenum. Occasionally when the aircraft shuts off the use of the high pressure bleed air, the bleed air is diverted through the surge plenum 99 into the exhaust system to prevent compressor surge. This quasi-radial dumping of surge bleed air in the mixing plane just downstream of the APU nozzle tends to reduce cooling air flow at the oil cooler.
A hard firewall bulkhead 6 located forward of the ITM 100 separates the APU compartment from the ITM 100 and defines a large forward end of a conical backing cavity 5 and also defines the inlet opening of the ITM 100. The backing cavity 5 is further defined by the skin of the tailcone 2 and by the narrow open aft end 7 of the tailcone. The tailcone 2 acts as the outer casing for the ITM 100 thereby defining the backing cavity 5 of the integrated dual use muffler. The muffler backing cavity 5 acts as both a muffler and as a surge bleed plenum for reducing discharge noise created while venting pressurized surge bleed air.
A cylindrical, porous exhaust liner 4 couples the oil cooler plenum 17 and an exhaust nozzle 13 of the APU 12 to the backing cavity 5 through the inlet opening in the firewall 6 and ultimately to the ambient atmosphere. The porous exhaust liner 4 is typically constructed of a perforated metal sheet, a porous metal wire mesh (e.g. Poroplate®), Feltmetal, any other suitable “linear” liner, or a suitable combination thereof.
The porous exhaust liner 4 has a first end 3 and extends through the inlet opening in the firewall bulkhead 6 to mechanically couple to an exhaust nozzle 13 of the APU 12 for receiving its exhaust flow. The porous exhaust liner 4 has a second end extending to the open aft end 7 of the tail cone 2 for porting mixed exhaust gas from the APU out of the open aft end of the tailcone. As used herein, the term “mixed exhaust gas” comprises APU cooling airflow, exhaust gas, and surge bleed air, as hereinafter described. The porous exhaust liner 4 is fluidly connected with the exhaust nozzle 13, the junction of which comprises an eductor in conjunction with of the APU exhaust nozzle 13, the oil cooler plenum 17 and the porous exhaust liner 4. The APU exhaust is the motive fluid for the eductor (13,17,4), which creates a reduced pressure in the oil cooler plenum 17 downstream of APU nozzle 13, also known as the mixing plane, thereby drawing cooling air through the APU compartment and over the APU oil cooler 10. The mixed exhaust is discharged through ITM 100 an out the open aft end 7.
Ideally, the APU exhaust flowing through the oil cooling eductor (13, 17, 4) can pull sufficient airflow through the APU compartment to provide a sufficient level of cooling by reducing the pressure at the air cooler plenum 17 by a small fraction (e.g., 1%) of the ambient atmospheric pressure. Unfortunately, surge bleed air resulting from unused APU compressor discharge air, is periodically vented from the bleed air system via a mounted surge discharge plenum 99 into the vicinity of the eductor (13, 17, 4) (e.g., downstream of the exhaust mixing plane). The periodic surge of bleed air increases the local pressure in the vicinity of the oil cooler plenum 17, thereby degrading the performance of the oil cooler 10. Further, the noise generated during the venting of the pressurized surge bleed air at the oil cooler plenum 17 is the dominant noise source from the APU 12 when it is not loaded by an onboard auxiliary system such as a heating or cooling system.
FIG. 1A is a graphic illustration of the pressure profile in the ITM 100. The Y axis is the absolute pressure that ranges from zero to ambient pressure of 14.7 PSI. The X axis is the axial distance from the APU nozzle 13 to the open aft end 7.
Looking at curve 150, at the APU nozzle 13 the pressure is significantly higher than 14.7 PSI as the APU exhaust exits the nozzle creating the motive fluid for the oil cooler air eductor (13, 17, 4). At the, oiler cooler plenum 17, a pressure drop is created by the eductor effect of the APU exhaust flow. This pressure drop is typically in the 1-2% range, which causes an absolute pressure drop of approximately 0.147-0.294 PSI. The pressure drop causes the APU cooling air to flow from the APU compartment across the APU oil cooler 10. As the mixed air flow travels down the porous liner 4, it rises to ambient pressure near the open aft end 7, which is in communication with an ambient atmosphere 1.
Looking at the curve 200, surge bleed air exiting the surge bleed air plenum 99 located near the oil cooler plenum 17 periodically adds air volume thereby increasing back pressure in the oil cooler plenum 17. Thus, during periods when bleed air is surging, little or no cooling air passes through the oil cooler 10, thereby degrading its effectiveness.
Further, attempts have been made to attenuate surge bleed air venting noise from APU engine installations by ducting the surge bleed air into a conventional APU turbine engine muffler located in the aircraft tailcone. The results, however, have been less than satisfactory because such conventional mufflers are small relative to the volume of the tailcone, provide insufficient noise attenuation and are relatively heavy, making them unsuitable for small, lightweight aircraft.
Accordingly, it is desirable to provide an ITM and method for using the same. It is also desirable to maintain effectiveness of the APU exhaust eductor (13, 17, 4), minimize its weight and complexity, improve oil cooler performance, and decrease surge bleed noise. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.